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The information in this profile may be out-of-date. It was last revised in 1996. EXTOXNET no longer updates this information, but it may be useful as a reference or resource.

Please visit the National Pesticide Information Center (NPIC) to find updated pesticide fact sheets. If you don't find a fact sheet related to your question, feel free to call 1-800-858-7378. NPIC is open five days a week from 8:00am to 12:00pm Pacific Time.

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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised June 1996

Metribuzin

Trade and Other Names: Trade names include Bay 94337, Bay DIC 1468, Lexone, Sencor, Sencoral, and Sencorex.

Regulatory Status: Metribuzin is a slightly toxic compound in EPA toxicity class III. It is a General Use Pesticide (GUP). Products containing metribuzin must bear the Signal Word CAUTION.

Chemical Class: triazine

Introduction: Metribuzin is a selective triazine herbicide which inhibits photosynthesis of susceptible plant species. It is used for control of annual grasses and numerous broadleaf weeds in field and vegetable crops, in turfgrass, and on fallow lands. Metribuzin is available as liquid suspension, water dispersible granular, and dry flowable formulations.

Formulation: Metribuzin is available as liquid suspension, water dispersible granular, and dry flowable formulations.

Toxicological Effects:

• Acute toxicity: Metribuzin is slightly toxic via the oral route, with reported oral LD50 values of 1090 to 2300 mg/kg in rats, 700 mg/kg in mice and 245 to 274 mg/kg in guinea pigs [6,15]. It is practically nontoxic dermally, with a dermal LD50 of 20,000 mg/kg in rabbits [6,15]. The 4-hour inhalation LC50 for metribuzin in rats is greater than 0.65 mg/L, indicating moderate toxicity via the inhalation route [15]. Metribuzin has been shown not to irritate the skin or eyes of rats, rabbits, guinea pigs, or human volunteers [15,32]. Effects of high acute exposure in metribuzin poisoned rats included narcosis (stupor) and labored breathing. Deaths occurred within 24 hours, and survivors recovered slowly without permanent effects [33].
• Chronic toxicity: No ill effects were observed in dogs fed dietary doses of 12.5 mg/kg/day for 3 months [33]. No effects were apparent in rats receiving 2.5 mg/kg/day over 3 months, but doses of 25 and 75 mg/kg/day caused enlarged livers and thyroid glands [33]. In 2-year feeding studies with rats and dogs, results showed no observable effects at doses of 5 mg/kg/day in rats and 2.5 mg/kg/day in dogs. Reduced weight gain, an increase in the number of deaths, blood chemistry changes, and liver and kidney damage were observed in a 2-year study in which dogs were given 1500 ppm or 37.5 mg/kg/day of metribuzin [34].
• Reproductive effects: Doses of 15, 45, or 135 mg/kg/day of technical metribuzin were administered by gavage to rabbits on days 6 through 18 of pregnancy. No effects on the mothers were observed at a dose of 45 mg/kg, but 135 mg/kg lowered maternal weight gain [35]. No effects on the fetuses were observed at any of the doses tested [35]. A three-generation study in rats at doses of up to 15 mg/kg/day (the highest dose tested), showed no influence on reproduction [32,36]. Metribuzin does not cause reproductive effects.
• Teratogenic effects: In rats, reduced fetal body weights were seen at doses of 70 mg/kg/day, and developmental delays were observed at doses of 200 mg/kg/day [15]. Metribuzin did not show teratogenic activity in rabbits at doses of up to 85 mg/kg/day, but did decrease weight gain in offspring [15]. These data suggest that metribuzin is unlikely to cause teratogenic effects in humans under normal circumstances.
• Mutagenic effects: Tests on live animals and on tissue cultures have shown that metribuzin has no mutagenic activity [15,32].
• Carcinogenic effects: There were no indications of carcinogenic effects in rats receiving dietary doses of up to 15 mg/kg/day for 2 years, nor in mice fed up to about 380 mg/kg/day for 2 years [32]. These data suggest that metribuzin is not carcinogenic.
• Organ toxicity: In single high dose studies, metribuzin appears to depress the central nervous system. Other studies indicate that the target organs of metribuzin are the thyroid gland and the liver.

• Fate in humans and animals: After metribuzin is absorbed, it is rapidly distributed in the body and excreted unchanged in the urine [6]. In mammals, 90% elimination occurs within 96 hours, about equally distributed between the urine and feces [6].

Ecological Effects:

• Effects on birds: Data indicate that metribuzin is moderately to slightly toxic to birds. The acute oral LD50 values are about 100 to 200 mg/kg in bobwhite quail, mallard ducks, and Japanese quail [6,15]. The reported 5- to 8-day dietary LC50 values for these species are all greater than 4000 ppm [6,15].
• Effects on aquatic organisms: Metribuzin is slightly toxic to fish. The 96-hour LC50 is 64 to 76 mg/L in rainbow trout, 80 mg/L in bluegill sunfish, and greater than 10 mg/L in goldfish [6,15]. The reported 48-hour LC50 in Daphnia magna is 4.5 mg/L, indicating similar toxicity [15]. The 96-hour LC50 in marine/estuarine shrimp is 48.3 mg/L [34].
• Effects on other organisms: It is nontoxic to bees [6]. Metribuzin may be phytotoxic to non-target plant species [6,15].

Environmental Fate:

• Breakdown in soil and groundwater: Metribuzin is of moderate persistence in the soil environment [20]. The half-life of metribuzin varies according to soil type and climatic conditions. Soil half-lives of 30 to 120 days have been reported; a representative value may be approximately 60 days [20]. Metribuzin is poorly bound to most soils and soluble in water, giving it a potential for leaching in many soil types [20]. Soil mobility is affected by many site-specific variables, including the amount of soil organic matter, particle size distribution, porosity, rainfall, and application rates. Metribuzin has been detected in Ohio rivers and Iowa wells and groundwater [35,34]. The major mechanism by which metribuzin is lost from soil is microbial degradation. Losses due to volatilization or photodegradation are not significant under field conditions [3,6].
• Breakdown in water: The half-life of metribuzin in pond water is approximately 7 days [6]. If present, metribuzin would most likely be found in the water column rather than the sediment, due to its low binding affinity and high water solubility.
• Breakdown in vegetation: Metribuzin is absorbed through the leaves when plants are given surface treatment, but the primary route for uptake is through the root system. From the roots, it is translocated upward, becoming concentrated in the roots, stems, and leaves of treated plants. In non-susceptible plants it is deaminized to more water-soluble conjugates; in susceptible plants it is not metabolized and disrupts phosynthesis in the chloroplast [15].

Physical Properties:

• Appearance: Metribuzin is a white, crystalline solid with a slightly sharp, sulfurous odor [6].
• Chemical Name: 4-amino-6-tert-butyl-4,5-dihydro-3-methyltio-1,2,4-triazin-5-one [6]
• CAS Number: 21087-64-9
• Molecular Weight: 214.29
• Water Solubility: 1050 mg/L [6]
• Solubility in Other Solvents: s. in aromatic and chlorinated hydrocarbon solvents, including dimethyl formamide, cyclohexane, methanol, benzene, ethanol, xylene, and kerosene [6]
• Melting Point: 125-126.5 C [6]
• Vapor Pressure: 0.058 mPa @ 20 C [6]
• Partition Coefficient: 1.6021 [6]
• Adsorption Coefficient: 60 (estimated) [20]

Exposure Guidelines:

• ADI: Not Available
• MCL: Not Available
• RfD: 0.025 mg/kg/day [26]
• PEL: Not Available
• HA: 0.2 mg/L (lifetime) [35]
• TLV: 5 mg/m3 (8-hour) [36]

Basic Manufacturer:

DuPont Agricultural Products
Walker's Mill, Barley Mill Plaza
P.O. Box 80038
Wilmington, DE 19880-0038

• Phone: 800-441-7515
• Emergency: 800-441-3637
  

References:

References for the information in this PIP can be found in Reference List Number 8

DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide product labeling or other regulatory requirements. Please refer to the pesticide product labeling.